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Early Pleistocene Oceanographic Changes around the Pacific Side of Japan based on Oxygen Isotope Analysis and Calcareous Nannofossil Assemblages

Daisuke KUWANOYoshimi KUBOTAKanako MANTOKUKoji KAMEO

Division of Earth and Environmental Sciences, Graduate School of Science and Engineering, Chiba University,JapanDepartment of Geology and Paleontology, National Museum of Nature and Science,JapanDepartment of Earth Sciences, Faculty of Science, Chiba University,Japan

摘要:The Mid-Pleistocene Transition(MPT) is the well-known interval(approximately 1250–700 ka) that Earth’s climate cycles changed from 41 ky to quasi-100 ky rhythms(e.g., Elderfield et al., 2012).Asymmetrical climate fluctuations between the glacial and interglacial times established during this interval(Head and Gibbard, 2015).The Kazusa Group, distributed in the central part of the Pacific side of the Japanese archipelago, is the Lower–Middle Pleistocene forearc basin-fill sediments.The Group is suitable for high-resolution paleoceanographic researches because its sedimentation rate is very high(ca.1–2 m/ky on average)(Kazaoka et al., 2015).This study will discuss sea-surface environmental changes during the Early Pleistocene around the central part of the Pacific side of Japan based on calcareous nannofossil assemblages and oxygen isotope analysis.We analyzed stable oxygen isotopes of a planktonic foraminifera Globorotalia inflata and benthic foraminifera Uvigerina spp., Bulimina spp., and Bolivinita quadrilatera.Oxygen isotopic measurements were conducted using MAT253 with a Kiel IV carbonate.At the same time, relative abundances of calcareous nannofossils were examined based on the usual counting method, and also separate counts for subordinate taxa, except for Gephyrocapsa, Reticlofenestra, Florisphaera, and Pseudoemiliania, were applied to obtain data for environmental changes.The average value of benthic δ18O is approximately 0.6 ‰ smaller than those of the LR04 stack.Based on the graphic correlation with the standard stack curve by LR04(Lisiecki and Raymo, 2005), Marine Isotope Stages(MIS) 41 to 36 were recognized in the examined section.Sequential fluctuations of planktonic δ18O and relative abundances of Florisphaera profunda(stratified, warm, and offshore index) resembles to those of benthic δ18O.It means that the glacial-interglacial climatic changes controlled the movement of the past Kuroshio front.Sudden change in calcareous nannofossils, dominances from a cold-water taxon, Coccolithus pelagicus, to a warm water taxon, Umbilicosphaera sibogae, was observed within ten ky immediately after MIS 38/37 boundary.Because planktonic δ18O values concordantly became the largest in this interval, the rapid warming of the surface to subsurface ocean environments was suggested.In the lower MIS 36, subsurface and bottom water masses may be slightly warm due to smaller values of planktonic and benthic δ18O profiles.
会议名称:

第一届亚洲古生物学大会—暨中国古生物学会成立90周年纪念

会议时间:

2019-11-17

会议地点:

中国北京

  • 专辑:

    理工A(数学物理力学天地生)

  • 专题:

    生物学; 海洋学; 地质学

  • 分类号:

    Q91;P736.22

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